Cans that serve as containers for beverages and foods are useful for storing the contents over a long period of time and are therefore used all over the world. Cans are roughly classified into the following two types: a two-piece can obtained by subjecting a metal sheet to drawing, ironing, stretching and bending to integrally form a can bottom and a can body and then joining the can body with a top lid by seaming; and a three-piece can obtained by machining a metal sheet into a tubular shape, welding the tubular metal sheet by a wire seam process to form a can body, and then joining the opposite ends of the can body separately with lids by seaming.
Conventionally, a tin-plated steel sheet (so-called tin plate) has been widely used as a steel sheet for cans. Nowadays, however, an electrolytic chromate treated steel sheet (hereinafter also called tin free steel (TFS)) having a chromium metal layer and a hydrated chromium oxide layer costs much less and has better paint adhesion than tin plates and is therefore expanding its range of application.
In connection with reduction in washing waste liquid and CO2 for environmental reasons, cans using a steel sheet laminated with an organic resin film such as PET (polyethylene terephthalate) is drawing attention as an alternative technique that enables a coating process and a baking process to be omitted, and also in this context, the use of TFS having excellent adhesion to an organic resin film is expected to continuously expand.
Meanwhile, since TFS is inferior to a tin plate in weldability, a hydrated chromium oxide layer which is an insulating coating at the surface layer is mechanically polished and removed immediately before welding to thereby make welding possible at present.
In industrial production, however, there are many problems in that, for instance, metal powder generated through polishing may be mixed in the contents, a burden of maintenance such as cleaning of can manufacturing equipment increases, and the risk of a fire caused by metal powder increases.
To address those issues, a technique of welding TFS without polishing is proposed by, for instance, JP 63-186894 A. In the technique disclosed by JP 63-186894 A, anodic electrolysis treatment is carried out between prior-stage and posterior-stage cathodic electrolysis treatments to thereby form a number of defect portions in a chromium metal layer, and then chromium metal is formed into a shape of granular protrusions through the posterior-stage cathodic electrolysis treatment. According to that technique, in welding, the granular protrusions of chromium metal destroy a hydrated chromium oxide layer that is a factor inhibiting welding at the surface layer, thereby reducing contact resistance and improving weldability.
We studied steel sheets for cans specifically described in JP 63-186894 A and found that, in some cases, they had poor surface appearance.
It could therefore be helpful to provide a steel sheet for cans having excellent surface appearance and a method of manufacturing the same.